This is a special case of Shotgun Surgery, discussed earlier.
The hierarchies probably grew in parallel, a class and its pair being needed at the same time. As usual, it probably wasn't bad at first, but after two or more pairs get introduced, it becomes too complicated to change one thing. (Often both classes embody different aspects of the same decision.)
What to Do
Reduces duplication. May improve communication. May reduce size.
None identified. (This smell may happen along the way from improving a particularly tangled situation.)